P/E Ratio — Price-to-Earnings Ratio Explained

The P/E ratio is the most quoted number in equity markets. It divides the current share price by earnings per share (EPS) over the most recent twelve months, producing a ratio that tells you how much you are paying per unit of current earnings.

The Calculation

P/E Ratio = Share Price / Earnings Per Share (EPS)

A P/E of 20 means investors are paying $20 for every $1 of annual earnings. Alternatively, if earnings were to remain constant and were all paid out as dividends, it would take 20 years to recoup the purchase price — which is why the P/E ratio is sometimes called the "earnings multiple."

Trailing vs. Forward P/E

• Trailing P/E (TTM): Uses earnings from the last twelve months. Backward-looking but based on actual reported numbers.
• Forward P/E: Uses analyst estimates of earnings for the next twelve months. More forward-looking but dependent on estimate accuracy.

When investors or media quote "the P/E", they typically mean trailing P/E unless specified otherwise.

What a High or Low P/E Means

A high P/E means investors expect earnings to grow substantially in the future — they are paying a premium for future earnings that are not yet reflected in current numbers. A low P/E suggests the opposite: investors expect slow growth, declining earnings, or they are uncertain about the company's future.

Context determines everything. A P/E of 30 is low for a fast-growing software company. A P/E of 12 is high for a slow-growing utility with regulatory risk. Comparing P/E ratios across sectors or time periods without context leads to wrong conclusions.

The Limitations of P/E

The P/E ratio tells you what the market is paying for current earnings. It does not tell you:

• Whether those earnings are high-quality (cash-backed) or accounting artefacts
• Whether the business model that generates them is durable
• Whether the company has pricing power, or whether margins are at peak and likely to compress
• What the competitive dynamics of the industry look like
• How much debt sits behind the equity

A company can have a low P/E because it is cheap — or because the market correctly identifies that its earnings are about to decline. This is the "value trap": buying on a low P/E only to watch earnings fall, leaving the P/E ratio unchanged or higher than when you bought.

Cyclically-Adjusted P/E (CAPE / Shiller P/E)

To address the volatility of earnings through economic cycles, Robert Shiller developed the CAPE ratio, which averages inflation-adjusted earnings over 10 years. This is more useful for comparing valuations across market cycles but less useful for individual company analysis where 10-year earnings averages are often meaningless.